1 . Field of the Invention
The present invention relates to a zoom lens and more particularly to a zoom lens with great zoom ratio.
2 . Related Background Art
Zoom lenses are going mainstream as photographing systems used for still cameras and video cameras. Also, cameras incorporated with an autofocus function are going mainstream.
Particularly, as technology on lens barrels has advanced in recent years, various zoom lenses in which the zoom ratio is increased by the use of a so-called multiple zoom lens with more than three movable lens units have been proposed.
Generally, in multiple zoom lenses, since the degrees of freedom in the selection of locus of each lens unit increase during moving of the lens units from a wide-angle end to a telephoto end, the degrees of freedom in aberration correction increase. Also, the number of lens units burdened with the refractive power varying operation increases, so that it is possible to aim at equalizing the burdens of the lens units in the refractive power varying operation, resulting in achievement of high zoom ratio and high performance.
In almost all the zoom lenses used for the above-mentioned cameras, focusing on a near distance object is performed by shifting one or a few lens units along the optical axis among the lens units to be shifted along the optical axis in the refractive power varying operation, i.e., movable variable refractive power lens units. Also, in order to achieve miniaturization of the lens system and simplicity of the drive mechanism of the focusing lens unit, the focusing lens unit is needed to be lightweight and its shift amount in the near distance focusing is required to be small generally.
For example, in a so-called four-unit focal zoom lens, near distance focusing is performed by means of a first lens unit disposed nearest to an object, and its entire lens length is not varied in a refractive power varying operation. That is, the amount of shift of the first lens unit as the focusing lens unit is constant without depending on variation in effective focal length. However, the first lens group has a larger diameter than the other lens units. Therefore, when aiming at miniaturizing the lens system, the first lens unit is not suitable for the focusing lens unit.
Alternately, there are a rear focus (RF) method and an inner focus (IF) method, wherein near distance focusing is performed by shifting a lens unit disposed nearer to an image surface than a first lens unit. When these methods are adopted, a lens unit with a small diameter can be selected as the focusing lens unit, which therefore is suitable for achieving miniaturization of the lens system.
However, in the rear focus (RF) method or the inner focus (IF) method, the amount of shift of the focusing lens unit is determined depending on both variation in photographing distance and variation in effective focal length of the lens system. That is, even at the same photographing distance, the amount of shift of the focusing lens unit is varied depending on variation in effective focal length.
The amount of shift of the focusing lens unit constituting a portion of the movable variable refractive lens units is expressed by the sum of an amount of shift due to its refractive power varying operation and an amount of shift due to its near distance focusing operation. Thus, when the focusing lens unit is a variable magnification-focusing lens unit to be shifted both in the refractive power varying operation and in the focusing operation, it is difficult to perform drive control of the variable magnification-focusing lens unit by shifting the lens unit along a cam groove in response to rotational movement of the lens barrel, i.e., by utilizing a mechanically-driven cam.
Hence, there is an increase in the number of zoom lenses in which the shift of a variable magnification power-focusing lens unit due to a refractive power varying operation and the shift thereof due to a near distance focusing operation are controlled electrically simultaneously by supplying an electric signal corresponding to the amount of shift of the lens unit to shift the lens unit a predetermined amount by means of a stepping motor and a rotating screw i.e., by utilizing an electrically-driven cam.
However, in such conventional zoom lenses, at the time of the manufacture, inconvenience occurs in that a back focus of the lens system needs to be adjusted while moving the variable magnification-focusing lens unit electrically.